DESCRIPTION (Applicant's Abstract): This revised R01 application is directed at establishing the role of aberrant B29 (Ig-beta, CD79b) expression and function in human chronic lymphocytic leukemia (B-CLL). While B-CLL is the most common form of leukemia in the United States and Europe, the genetic origins of is leukemia have not been established. B-CLL cells prevalently exhibit defects in B-cell receptor surface BCR display and signaling consistent with mutations affecting critical BCR members (i.e., IgH, IgL, mb-1 or B29). We recently discovered that leukemic B-CLL cells from >80 percent of patients studied exhibited diminished or undetectable surface B29 which correlated with surface Ig expression. Such B-CLL cells either failed to express B29 mRNA or expressed B29 mRNA with a range of mutations predicted to diminish or block the assembly, surface display or signaling of BCR complexes. The effects of such B29 mutations on BCR display a function could produce the unresponsive and noncycling state that underlies he prolonged survival of B-CLL cells. The molecular immunology studies proposed here are aimed at: 1) identifying the mechanisms leading to the extinction of B29 gene expression, 2) determining the functional consequences of B29 mutations on surface BCR assembly and function, and 3) testing our new proposal that the widespread B29 mutations in B-CLL are generated by somatic hypermutation. This new direction replaces the mutant B29 transgenic mouse model studies deemed to be risky in the previous review. The establishment of long-term cultures of B-CLL cells and the generation and successful use of vaccinia and adenovirus gene transfer vectors for efficient B29 gene replacements in B-CLL cells are critical technical developments supporting these ongoing studies. The new studies on somatic hypermutation directly test a novel mechanism for the B29 mutations in B-CLL. Somatic hypermutation of Ig genes in B-CLL patients is reported to be correlated with better clinical prognosis. Demonstration of somatic hypermutation in B29 genes would generate new insights into genetic origins of B-CLL and provide a reliable clinical test for predicting outcome and selecting appropriate therapy.